1. Field of the Invention
An aspect of the present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus discharging a printing paper at a stable speed.
2. Description of the Related Art
Generally, an image forming apparatus includes a paper providing unit, a developing unit, a fixing unit and a paper discharging unit. The paper providing unit provides a printing paper to the developing unit, which selectively applies developer to the printing paper, thereby forming an image. The fixing unit fixes the applied developer to the printing paper. The paper discharging unit receives the printing paper on which the image is formed and discharges the paper having the formed image to the outside of the image forming apparatus. Although the term “printing paper” is used as a convenience in describing aspects of the present invention, the present invention is applicable to printable media other than paper, such as for example, plastic sheets.
The developing unit includes a photosensitive body, a light scanning unit, a developing roller and a transferring roller. When the light scanning unit transfers light containing printing image information to the photosensitive body, an electrostatic latent image is formed by a potential difference on the photosensitive body. The developing roller provides the developer to the electrostatic latent image on the photosensitive body and develops the electrostatic latent image, and the transferring roller transfers a development image formed on the photosensitive body to the printing paper.
In order to improve printing speed, the printing paper is transported in the apparatus at high speed. All of the paper providing unit, the developing unit, the fixing unit and the paper discharging unit transport the printing paper at high speed. However, the printing speed should decelerate to a predetermined speed or less at the moment the printing paper is last discharged to the outside of the apparatus. Therefore, the paper discharging unit transports the printing paper at variable speed.
FIG. 1 is a schematic view of a conventional image forming apparatus. The image forming apparatus 100 includes a paper providing roller part 160, a developing part 110, a fixing roller part 120, a transporting roller part 140 and a paper discharging roller part 130. The paper providing roller part 160 includes a pickup roller 162 picking up a printing paper (P) from a paper providing tray (not shown) and a feeding roller 164 feeding the printing paper (P) to the developing part 110.
The developing part 110 selectively applies developer to the printing paper (P) according to a developing process. The fixing roller part 120 includes a pair of fixing rollers and receives the printing paper (P) to which the developer is applied from the developing part 110 and fixes the developer to the printing paper (P), thereby forming an image.
The transporting roller part 130 includes a transporting roller 142 receiving the printing paper (P) from the fixing roller part 120 and transporting the printing paper (P) to the paper discharging roller part 130 and a transporting roller driving part 144 driving the transporting roller 142. The transporting roller driving part 144 drives the transporting roller 142 in order to transport the printing paper (P) at the same speed as the fixing roller part 120 transports the printing paper (P), using a same driving motor as the fixing roller part 120 so as not to influence the speed at which the fixing roller part 120 fixes the printing paper (P).
The paper discharging roller part 130 includes a paper discharging roller 132 discharging the printing paper (P) to a paper storing part 150 and a paper discharging roller driving part 134 driving the paper discharging roller 132. The printing paper (P) is orderly transported by the paper providing roller part 160, the developing part 110, the fixing roller part 120 and the transporting roller part 140 so as to be printed at high speed.
When the paper discharging roller part 130 and the transporting roller part discharge the printing paper (P) to the paper storing part 150 at high speed, a following printing paper (P) is stacked before a former printing paper (P) is settled and the printing papers (P) may not be properly stacked in the paper storing part 150. Therefore, after receiving the printing paper (P) from the transporting roller part 140 at high speed, the paper discharging roller part 130 discharges the printing paper (P) at a predetermined decelerated speed before discharging the printing paper (P) to the paper storing part 150, and then transports the following printing paper (P) at the original high speed.
FIG. 2 is a graph showing a transporting speed pattern of the paper discharging roller part in FIG. 1. A horizontal axis shows time and a vertical axis shows a transporting speed of the printing paper (P) of the paper discharging roller part 130. Time ‘a’ shows a time at which a lower part of the printing paper (P) leaves the fixing roller part 120, time ‘b’ shows a time at which the lower part of the printing paper (P) leaves the transporting roller part 140, and time ‘c’ shows a time at which the lower part of the printing paper leaves the paper discharging roller part 130. Line Vp shows the transporting speed pattern of the image forming apparatus 100.
The fixing roller part 120 and the transporting roller part 140 regularly transport the printing paper P at a high fixing speed V1. The paper discharging roller part 130 continues transporting the printing paper (P) at V1, the same speed as the fixing speed V1, until the time ‘b’ so as not to disturb the fixing roller part 120 in fixing the printing paper. The paper discharging roller part 130 decelerates down to a discharging speed V3 at the time ‘c’ and discharges the printing paper (P), and then returns the discharging speed to a fixing speed V1. In order to increase an interval between the printing papers (P) so that the former printing paper is not contacted with the following printing paper when the printing paper is deceleratedly transported between time ‘b’ and time ‘c’, the paper discharging roller part 130 acceleratedly transports the printing paper (P) up to a predetermined accelerated speed V2 after time ‘b’, and thereafter deceleratedly transports the printing paper (P) down to the discharging speed V3.
However, the conventional image forming apparatus 100 may not correctly discharge the printing paper (P) since the printing speed decelerates at a sufficiently stable speed as the printing speed becomes higher-speed. Since the fixing speed V1 becomes an even higher-speed as the printing speed becomes high-speed, a difference in transporting speed at which the paper discharging roller part 130 should decelerate from time ‘b’ to time ‘c’ becomes large, and a time interval between time ‘b’ and the time ‘c’ may become short. Therefore, the paper discharging roller part 130 accelerates the transporting speed between the time ‘b’ and the time ‘c’, and then may not decelerate the transporting speed back to stable speed. Accordingly, the printing paper (P) is discharged and transported at a speed which is not sufficiently decelerated to be stable, thereby the printing paper is incorrectly stacked.
To discharge the printing paper at stable speed, the transporting roller part 140 may be spaced from the paper discharging roller part 130 in order to extend the interval between the time ‘b’ and the time ‘c’. However, the distance between the transporting roller part 140 and the paper discharging roller part 130 may not be more than the maximum length of the printing paper (P). Otherwise, the printing paper (P) may not be stably transported. Increasing a transporting interval between the printing papers may obtain time to decelerate the transporting speed, however, increasing the transporting interval makes the printing speed slower.